773 research outputs found
Edge spin accumulation: spin Hall effect without bulk spin current
Spin accumulation in a 2D electron gas with Rashba spin-orbit interaction
subject to an electric field can take place without bulk spin currents (edge
spin Hall effect). This is demonstrated for the collisional regime using the
non-equilibrium distribution function determined from the standard Boltzmann
equation. Spin accumulation originates from interference of incident and
reflected electron waves at the sample boundary.Comment: 4 pages, 3 figure
Spin relaxation of two-dimensional holes in strained asymmetric SiGe quantum wells
We analyze spin splitting of the two-dimensional hole spectrum in strained
asymmetric SiGe quantum wells (QWs). Based on the Luttinger Hamiltonian, we
obtain expressions for the spin-splitting parameters up to the third order in
the in-plane hole wavevector. The biaxial strain of SiGe QWs is found to be a
key parameter that controls spin splitting. Application to SiGe field-effect
transistor structures indicates that typical spin splitting at room temperature
varies from a few tenth of meV in the case of Si QW channels to several meV for
the Ge counterparts, and can be modified efficiently by gate-controlled
variation of the perpendicular confining electric field. The analysis also
shows that for sufficiently asymmetric QWs, spin relaxation is due mainly to
the spin-splitting related D'yakonov-Perel' mechanism. In strained Si QWs, our
estimation shows that the hole spin relaxation time can be on the order of a
hundred picoseconds at room temperature, suggesting that such structures are
suitable for p-type spin transistor applications as well
Transverse Spin-Orbit Force in the Spin Hall Effect in Ballistic Semiconductor Wires
We introduce the spin and momentum dependent {\em force operator} which is
defined by the Hamiltonian of a {\em clean} semiconductor quantum wire with
homogeneous Rashba spin-orbit (SO) coupling attached to two ideal (i.e., free
of spin and charge interactions) leads. Its expectation value in the
spin-polarized electronic wave packet injected through the leads explains why
the center of the packet gets deflected in the transverse direction. Moreover,
the corresponding {\em spin density} will be dragged along the transverse
direction to generate an out-of-plane spin accumulation of opposite signs on
the lateral edges of the wire, as expected in the phenomenology of the spin
Hall effect, when spin- and spin- polarized packets
(mimicking the injection of conventional unpolarized charge current) propagate
simultaneously through the wire. We also demonstrate that spin coherence of the
injected spin-polarized wave packet will gradually diminish (thereby
diminishing the ``force'') along the SO coupled wire due to the entanglement of
spin and orbital degrees of freedom of a single electron, even in the absence
of any impurity scattering.Comment: 5 pages, 4 color EPS figures; 2 new figures and expanded discussion
on the sign of spin Hall quantities. To appear in Phys. Rev. B 72 (2005
Electron spin relaxation in graphene: the role of the substrate
Theory of the electron spin relaxation in graphene on the SiO substrate
is developed. Charged impurities and polar optical surface phonons in the
substrate induce an effective random Bychkov-Rashba-like spin-orbit coupling
field which leads to spin relaxation by the D'yakonov-Perel' mechanism.
Analytical estimates and Monte Carlo simulations show that the corresponding
spin relaxation times are between micro- to milliseconds, being only weakly
temperature dependent. It is also argued that the presence of adatoms on
graphene can lead to spin lifetimes shorter than nanoseconds.Comment: 5 pages, 4 figure
Random walk approach to spin dynamics in a two-dimensional electron gas with spin-orbit coupling
We introduce and solve a semi-classical random walk (RW) model that describes
the dynamics of spin polarization waves in zinc-blende semiconductor quantum
wells. We derive the dispersion relations for these waves, including the
Rashba, linear and cubic Dresselhaus spin-orbit interactions, as well as the
effects of an electric field applied parallel to the spin polarization
wavevector. In agreement with fully quantum mechanical calculations [Kleinert
and Bryksin, Phys. Rev. B \textbf{76}, 205326 (2007)], the RW approach predicts
that spin waves acquire a phase velocity in the presence of the field that
crosses zero at a nonzero wavevector, . In addition, we show that the
spin-wave decay rate is independent of field at but increases as
for . These predictions can be tested experimentally by
suitable transient spin grating experiments
Resonance-like electrical control of electron spin for microwave measurement
We demonstrate that the spin-polarized electron current can interact with a
microwave electric field in a resonant manner. The spin-orbit interaction gives
rise to an effective magnetic field proportional to the electric current. In
the presence of both dc and ac electric field components, electron spin
resonance occurs if the ac frequency matches with the spin precession frequency
that is controlled by the dc field. In a device consisting of two
spin-polarized contacts connected by a two-dimensional channel, this mechanism
allows electrically tuned detection of the ac signal frequency and amplitude.
For GaAs, such detection is effective in the frequency domain around tens of
gigahertz.Comment: 10 pages, 2 figure
Suppression of the D'yakonov-Perel' spin relaxation mechanism for all spin components in [111] zincblende quantum wells
We apply the D'yakonov-Perel' (DP) formalism to [111]-grown zincblende
quantum wells (QWs) to compute the spin lifetimes of electrons in the
two-dimensional electron gas. We account for both bulk and structural inversion
asymmetry (Rashba) effects. We see that, under certain conditions, the spin
splitting vanishes to first order in k, which effectively suppresses the DP
spin relaxation mechanism for all spin components. We predict extended spin
lifetimes as a result, giving rise to the possibility of enhanced spin storage.
We also study [110]-grown QWs, where the effect of structural inversion
asymmetry is to augment the spin relaxation rate of the component perpendicular
to the well. We derive analytical expressions for the spin lifetime tensor and
its proper axes, and see that they are dependent on the relative magnitude of
the BIA- and SIA-induced splittings.Comment: v1: 5 pages, 2 figures, submitted to PRL v2: added 1 figure and
supporting content, PRB forma
Spin relaxation in a GaAs quantum dot embedded inside a suspended phonon cavity
The phonon-induced spin relaxation in a two-dimensional quantum dot embedded
inside a semiconductor slab is investigated theoretically. An enhanced
relaxation rate is found due to the phonon van Hove singularities. Oppositely,
a vanishing deformation potential may also result in a suppression of the spin
relaxation rate. For larger quantum dots, the interplay between the spin orbit
interaction and Zeeman levels causes the suppression of the relaxation at
several points. Furthermore, a crossover from confined to bulk-like systems is
obtained by varying the width of the slab.Comment: 5 pages, 4 figures, to apper in Phys. Rev. B (2006
Band structure and magnetotransport of a two-dimensional electron gas in the presence of spin-orbit interaction
The band structure and magnetotransport of a two-dimensional electron gas
(2DEG), in the presence of the Rashba (RSOI) and Dresselhaus (DSOI) terms of
the spin-orbit interaction and of a perpendicular magnetic field, is
investigated. Exact and approximate analytical expressions for the band
structure are obtained and used to calculate the density of states (DOS) and
the longitudinal magnetoresitivity assuming a Gaussian type of level
broadening. The interplay between the Zeeman coupling and the two terms of the
SOI is discussed. If the strengths and , of the RSOI and DSOI,
respectively, are equal and the factor vanishes, the two spin states are
degenerate and a shifted Landau-level structure appears. With the increase of
the difference , a novel beating pattern of the DOS and of the
Shubnikov-de Haas (SdH) oscillations appears distinctly different from that
occurring when one of these strengths vanishes
Spin polarization decay in spin-1/2 and spin-3/2 systems
We present a general unifying theory for spin polarization decay due to the
interplay of spin precession and momentum scattering that is applicable to both
spin-1/2 electrons and spin-3/2 holes. Our theory allows us to identify and
characterize a wide range of qualitatively different regimes. For strong
momentum scattering or slow spin precession we recover the D'yakonov-Perel
result, according to which the spin relaxation time is inversely proportional
to the momentum relaxation time. On the other hand, we find that, in the
ballistic regime the carrier spin polarization shows a very different
qualitative behavior. In systems with isotropic spin splitting the spin
polarization can oscillate indefinitely, while in systems with anisotropic spin
splitting the spin polarization is reduced by spin dephasing, which is
non-exponential and may result in an incomplete decay of the spin polarization.
For weak momentum scattering or fast spin precession, the oscillations or
non-exponential spin dephasing are modulated by an exponential envelope
proportional to the momentum relaxation time. Nevertheless, even in this case
in certain systems a fraction of the spin polarization may survive at long
times. Finally it is shown that, despite the qualitatively different nature of
spin precession in the valence band, spin polarization decay in spin-3/2 hole
systems has many similarities to its counterpart in spin-1/2 electron systems.Comment: 4 pages, 1 figure, to appear in Phys. Rev.
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